This invention relates to a process for preparing ethylene glycol esters by the catalyzed reaction of ethylene, oxygen and a carboxylic acid in the vapor phase.
Ethylene glycol esters are a well known class of organics having widespread use as solvents and plasticizers. Thus, for example, ethylene glycol diacetate is employed as a solvent for cellulose esters and ethers and is often included in resin, lacquer and printing ink formulations. The monoacetate ester of ethylene glycol is used as a solvent for nitrocellulose. Another principal use for ethylene glycol esters is as intermediates in the preparation of corresponding ethylene glycol, which is one of the most industrially important dihydric alcohols.
In view of the considerable commercial significance of the ethylene glycol esters, both as useful products in themselves and as intermediates in chemical synthesis, there is an ongoing need to provide processes for preparing the esters in the most economical manner possible.
Recent years have been an especially active period in the development of processes for obtaining ethylene glycol esters by the one-step catalyzed reaction of ethylene, oxygen and a carboxylic acid. The reaction, commonly referred to as oxyacylation, is usually carried out in the liquid phase at elevated temperature and superatmospheric pressure. Thus for the reaction of ethylene, oxygen and acetic acid to produce ethylene glycol diacetate, the overall chemical reaction can be considered to proceed in accordance with the reaction: ##EQU1##
In addition to diester, this reaction will also result in the production of some monoester. Depending in large measure upon the catalyst employed, reactions other than the aforesaid oxyacylation reaction can take place, and to the extent they reduce product yield, complicate recovery and separation techniques and increase raw material and production costs, they are undesirable. The alkylene glycol mono- and diesters are readily hydrolyzed to the glycols and to the carboxylic acid(s) employed in their production employing known and conventional techniques, e.g., hydrolysis by saponification with alkali, or by alcoholysis with acids or acidic ion exchanger.
Numerous proposals for liquid phase oxyacylation catalysts have been made. U.S. Pat. No. 2,519,754 employs as catalyst, a hydrohalide such as hydrobromic acid or an aliphatic halide such as methyl bromide. Snyder's process described in U.S. Pat. No. 2,701,813 employs certain metals or metal compounds such as silver, compounds of metals of the first transition group of the periodic system, particularly their salts, and salts of such heavy metals which are capable of existing in more than one oxidation state such as the acetates, stearates and naphthenates of cobalt, manganese, copper and the like. U.S. Pat. No. 3,479,395 to Huguet describes a process employing tellurium dioxide solubilized with a halide salt. Lutz, in U.S. Pat. No. 3,542,857 employs a cerium (III) or cerium (IV) salt which is soluble in the carboxylic acid component of the oxyacylation reaction medium. U.S. Pat. Nos. 3,668,239 and 3,789,065 to Kollar, and U.S. Pat. No. 3,907,874 to Harvey et al. each employ a source of metal cation such as that of tellurium and a source of bromine. U.S. Pat. Nos. 3,689,535 and 3,985,795 to Kollar and U.S. Pat. No. 3,872,164 to Schmidt each describe a process for preparing ethylene glycol esters by contacting ethylene, bromine or chlorine, or a compound of bromine or chlorine, and oxygen in the presence of a carboxylic acid and a variable metal cation such as antimony cation. Similarly, the Valbert (U.S. Pat. No. 3,715,388) and Hoch (U.S. Pat. No. 3,715,389) catalysts include bromine or a bromine compound and a source of metal cation such as arsenic or antimony cation. U.S. Pat. No. 3,770,813 to Kollar describes the use of a catalyst containing iodine or iodide anion and a heavy metal cation of atomic number 21-30 and 48. U.S. Pat. No. 3,778,468 employs a catalyst containing selenium cation and chlorine, bromine or a compound thereof. Gaenzler et al. U.S. Pat. No. 3,916,011 describes an oxyacylation catalyst which is a complex formed between a compound of titanium and a compound of lithium, beryllium, magnesium, calcium, boron, aluminum, silicon or phosphorus, or a complex formed between compounds of at least two of the elements boron, aluminum, silicon and phosphorus. The oxyacylation catalyst of Gaenzler et al. U.S. Pat. No. 3,981,908 contains a compound of boron, aluminum, silicon, phosphorus or a combination thereof, and a compound of iron, copper or a combination thereof. The Schmerling (U.S. Pat. No. 4,009,203) catalyst is the reaction product of a tin halide and a carboxylic acid.
Each of the aforesaid oxyacylation catalysts is subject to one or more disadvantages, either in regard to the complexity of the apparatus required to carry out the process and/or in regard to the degree of selectivity of the reaction of ethylene, oxygen and carboxylic acid for ethylene glycol ester. In the case of the known catalysts which are intended for use in the liquid phase, the added complexity of operating under liquid phase conditions is a decided drawback. It is recognized that liquid phase systems containing a halogen, oxygen and a carboxylic acid are extremely corrosive and require expensive corrosion-resistant equipment and intensive equipment maintenance.